1. Field of the Invention
The present invention in general relates to a compensation control circuit of horizontal scanning size(H-size) for a monitor, more specifically, to a compensation control circuit and compensation method for compensating the variation of the horizontal scanning size due to the change in anode voltage of a picture tube when adjusting the luminance of the frame.
2. Description of the Prior Art
A cathode ray tube monitor(hereafter "CRT monitor") is a common monitor apparatus such as monochrome/color televisions or monochrome/color computer monitors etc. The CRT monitor is composed of cathode ray tube (CRT) and its peripheral processing circuit. The axiom of the CRT is mainly making use of electron beam to hit the fluorescent materials to fluoresce. The peripheral circuit is applied for controlling the path and the speed of the electron beam. FIG. 1 (Prior art) shows the cross-sectional view of a general CRT. Referring to FIG. 1, a CRT is mainly composed of the front section (including: filament 10, cathode 12, grid 14, anode 16, and deflection yoke 18) used to generate and accelerate the electron beam, the middle section (including: cavity 20 and anode button 22) used to deflect and accelerate the electron beam, and the faceplate section (including: aluminum membrane 30, fluorescence membrane 32, and screen glass 34) used to actually fluoresce. The filament 10 is heated by the current to generate the thermal electrons which are shot from the cathode 12. The thermal electrons shot from the cathode 12 form the focusing electron beam 40 through the focusing effect of the grid. The anode 16 draws electron beam 40 to accelerate by the electrode characteristic. In actual applications, the anode in the CRT is mostly composed of a plurality of anode electrodes. And the voltage could be many thousand volts to many ten thousand volts. On the other hand, the deflection yoke 18 guides the electron beam 40 to change its moving direction by the electromagnetic characteristic. In actual applications, the deflection yoke is provided with two groups: horizontal deflection yoke(H-DY), and vertical deflection yoke(V-DY). These two groups of deflection yokes will respectively control the horizontal and the vertical direction of the movement of the electron beam according to the horizontal scanning control signal and the vertical scanning control signal. The electron beam 40 accelerated and deflected to a predetermined angle go on a rectilinear motion in the cavity 20 of the CRT, until landing the fluorescence membrane 22 of the faceplate to fluoresce. During advancing in the cavity 20, the electron beam is still drawn by the anode high voltage 22 to accelerate.
In the ideal CRT picture tube, the horizontal size of the showing screen is controlled by the corresponding horizontal deflection yoke. As FIG. 1 shows, the horizontal yoke 18 in the neck portion of the CRT controls the range of the deflection angle within the neck portion by its magnetic field intensity. Then, the electron beam 40 leaves the neck portion in a predetermined angle and goes on to accelerate directly. That is, in the ideal condition, the deflection angle of the electron beam 40 can be determined by the magnetic field intensity generated by the horizontal yoke 18. But in the actual applications, if the anode voltage changes, the deflection angle .theta. will change also.
When the electron beam 40 goes into the neck portion controlled by the deflection yoke 18, it is influenced by the magnetic field(its direction is outward the paper or inward the paper) generated by the deflection yoke 18, and pushed by the magnetic force F.sub.m =qv.times.B to shift in the deflection direction, wherein q denotes the charge, v denotes speed of the motion, and B the denotes the magnetic field generated by the deflection yoke. Besides, the deflection angle .theta. is basically determined by the horizontal shift position of the electron beam 40 due to the variation of the magnetic field in the neck portion. How the anode voltage influences the deflection angle .theta. of the electron beam 40 will be illustrated briefly as follows according to the above phenomenon.
When the anode voltage increases, it means the electron beam 40 is accelerated by the stronger magnetic field. Then when the electron beam 40 enters the neck portion controlled by the deflection yoke 18, the passing time becomes shorter because the speed is faster than that in the normal condition. Then the horizontal shift position due to the magnetic field B becomes smaller, that is, the deflection angle becomes smaller. Therefore, the range of the landing area in the screen becomes narrower, that is to say, the horizontal scanning size will be smaller than that in the normal condition. On the other hand, when the anode voltage decreases, the speed of the electron beam 40 becomes slower accordingly. Consequently, as the deflection angle .theta. becomes larger, the horizontal scanning size becomes larger than that in the normal condition. The relationship between the horizontal scanning size and the anode voltage described above can be verified by the motion model of the electron beam, and will not be further described here. According to the above description, the horizontal scanning size is inversely proportional to the anode voltage.
In the normal condition, the anode voltage maintains stable and will not be drifted. But in some practical conditions, the anode voltage is indeed changed by the parameters of the other circuits, resulting in the instability of the horizontal scanning size. For example, while tuning the luminance of the frame, the anode high voltage changes due to the change in the load condition.
The method of the prior art to solve the problem is to stabilize the voltage directly. For example, there is voltage stabilizing circuit dealing with high voltage in the general TV to keep the stability of the high voltage. But the cost of the high voltage stabilizing circuit is very expensive, not all the monitors will adopt the design. This is one of the disadvantages of the prior art. Hence, the present invention provides an effect and economic method to solve the problem described above.